1. Field of the Invention
The present invention relates generally to vertical cavity surface emitting lasers (VCSELs). More particularly, the present invention relates to VCSELs which provide independently definable current confinement and optical mode confinement.
2. Field of the Invention
As compared with conventional edge-emitting semiconductor lasers, vertical cavity surface emitting lasers (VCSELs) hold the promise of a number of desirable characteristics. For example, shorter cavity resonator VCSELs provide excellent mode selectivity and, therefore, narrower linewidths. Use of multi-layered distributed Bragg reflectors (DBRs) to form a cavity resonator perpendicular to the layers obviates the need for cleaving operations common to edge emitting lasers. The perpendicular orientation of the resonator also facilitates fabrication and wafer-level testing of individual lasers.
Two basic types of VCSEL designs are known to exist: one defines a current confinement region in a semiconductor DBR by means of an apertured, high resistivity ion implanted region, whereas the other defines the current confinement region by means of an apertured, high resistivity oxide layer.
In the ion-implanted approach, light ions (e.g., protons) are implanted at relatively deep depths within the VCSEL device (e.g., about 3 μm). However, due to ion straggle and other difficulties associated with deep ion implantation, this current guide must be relatively large (e.g., >10 μm). Both of these factors inhibit scaling the devices to smaller sizes. In addition, the ion-implanted VCSEL does not form any significant optical guiding; i.e., it does not provide refractive index guiding of transverse lasing modes, although there may be some gain guiding of the modes. As a result, these lasers typically have threshold currents >1 mA and operating currents >3 mA. Electrical power dissipation per laser is, therefore, at least several mW.
In contrast, the oxide confinement approach is scalable to much smaller dimensions (e.g., the current aperture may be as small as 3 μm), which allow for an order of magnitude decrease in both the threshold and operating currents. In addition, the apertured oxide layer also forms a refractive index guide which leads to transverse mode confinement, resulting in at least another factor of two reduction in these currents. Thus, the power dissipation per device can be reduced by at least a factor of twenty (to a fraction of a mW) compared to the ion implanted design.
However, oxide VCSELs have not yet proven to be as reliable as ion implanted VCSELs and may have a built-in stress problem. Moreover, the oxidation process is relatively unreproducible and hence is not conducive to high yields. More specifically, oxidation processes entail oxidizing a high-Al content Group III-V layer after being covered by other layers; i.e., the outer edges of the high Al-content layer are exposed to water vapor so that oxidation progresses inwardly over a relatively long lateral distance (e.g., 10 s per μm) and yet must be precisely stopped so as to leave a very small diameter (e.g., 3 μm) current guide unoxidized. This process entails controlling oxidation time, assuming knowledge of the oxidation rate of the high-Al content Group III-IV layer. However, this rate depends on many factors, including parameters of the process and dimensions and properties of the materials to be oxidized. Controlling all of these factors is very difficult.
Thus, a need remains in the art for a VCSEL design that provides for both current and optical confinement and yet is scaleable, reproducible, and amenable to array applications.